Retorts are used for the in-container preservation of foodstuffs, either for pasteurization or sterilization processes. In general, these machines use a combination of pressure and temperature to process packaged food according to a predefined schedule. Additional overpressure for pasteurizing or sterilizing certain containers of foodstuffs may be achieved in a spray retort, where water or another suitable processing fluid is sprayed from the top (and optionally also from the sides) of the vessel through the load of containers. The processing fluid is used as a media for transferring heat into and out of the food containers during the sterilization process. The processing fluid may be heated through external means, or alternatively steam may be directly injected into the vessel.
As can be appreciated, using water or another fluid (“processing fluid”) continuously in this manner adds thermal mass to the process cycle and requires heating and cooling energy above that which is required by the food containers themselves. For instance, every cycle the processing fluid must be heated from ambient temperature up to about 121° C. (approximately 250° F.) and pressurized (for the steam to reach that temperature). Thus, reducing the quantity of processing fluid required for the spray retort process reduces the quantity of energy and fluid utilities consumed per process cycle (i.e., steam consumption). In other words, a lower volume of processing fluid needs to be heated and cooled every cycle. A reduction in the quantity of energy and fluid utilities consumed per process cycle results in a lower cost per container for the process cycle.
However, the reduction in the retort fluid level or volume is limited by the ability of the retort pump to circulate a relatively constant volume of processing fluid at a substantially high, selected flow rate (which may be defined by the velocity of the fluid exiting the retort). The selected flow rate will depend on various factors, such as the size of the load inside the retort, the diameter of the retort, whether the retort is in the cook or cool process, and other factors. As non-limiting examples, a 6 basket 1400 mm diameter retort during the cool process may have a flow rate of around at least 700 gallons per minute (gpm) through the retort outlet. A 6 basket 1800 mm diameter retort during the cook process may have a flow rate of around 1500 gpm through the retort outlet. Larger retorts, larger loads inside the retorts, and other factors would require more flow within the retort. Thus, although the flow rates of a retort will vary, it can be appreciated that the flow rate required or selected within a retort is significantly higher than what gravity would otherwise provide to remove the fluid from the bottom of the retort.
The processing fluid at the bottom of the retort is drawn to the suction side of the pump at the selected flow rate from one or more openings (i.e., suction points) in the bottom of the retort. While the pump is circulating the suctioned fluid at the selected high flow rate, the level of fluid remaining within the retort is reduced at the suction points, and the fluid level remains higher further away from the suction points. As a result of this fluid level gradient, the initial processing fluid fill level (i.e., volume) in the retort prior to starting the pump must be sufficiently high (or at a minimum level) such that the processing fluid level above the suction points does not cause air or steam entrainment or cavitation of the pump, which would lead to significant loss of pump flow.
As an example, FIG. 1 depicts a first prior art retort 20 having a single opening or suction point 24. When the pump (not shown) is started and the processing fluid inside the retort is removed at the selected flow rate, the fluid level above the suction point 24 lowers relative to the fluid level at the ends of the retort 20. This significant fluid level gradient is created naturally by the processing fluid as it tries to return to a natural level. More specifically, the processing fluid at the high ends has more potential energy than the lower processing fluid near the suction point 24, and as a result, a flow of processing fluid goes from high to low. When all points along the fluid surface have the same potential energy, the surface is level and flow stops. With continual removal and replacement of processing fluid at the same high flow rate, however, the natural levelling does not occur and the gradient becomes essentially constant.
As more suction points 24 are added, as shown in the second prior art retort 30 of FIG. 2, the resulting significant gradients combine to create peaks and valleys, similar to a sine wave pattern. The frequency of the wave increases as more suction points are added, but the amplitude decreases because the flow rate is divided across the suction points. It can be appreciated that as the number of suction points approaches infinity, the amplitude will approach zero. In other words, the more suction points there are along the retort vessel, the more closely the fluid surface approximates a constant, even level. However, it is not practical to manufacture a retort vessel with such a large number of suction pipes to make the fluid surface level or even.
Previous attempts to minimize processing fluid volume and avoid pump cavitation or entrainment have been focused only on adding deflectors or diffuser geometries around the opening of the pipe. While such prior art designs have helped distribute the processing fluid at higher fluid volumes, they have not been sufficient to achieve a significantly lower volume of processing fluid within the retort.
Accordingly, a design that minimizes suction points while distributing the selected processing fluid suction flow (and therefore the processing fluid level) substantially evenly along the length of at least a portion of the retort is needed. Such a design would reduce or eliminate the significant gradient effect caused in prior art systems, allowing the processing fluid to be reduced to the minimum volume required to operate the retort spray process at the selected flow rate.